The present invention relates to a common-mode filter for suppressing common-mode noise in a signal line or the like and particularly to a small-size common-mode filter used in a high-speed differential signal interface such as a DVI (digital visual interface) or an HDMI (high definition multimedia interface).
There is also known a structure, as shown in FIG. 8, in which normal-mode impedance is reduced in such a manner that a pair of wires are bifilar-wound on a core portion of a drum type core to improve the degree of coupling of the wires, and that lead-out portions of the pair of wires are led out while coming close to each other to improve the degree of coupling of the wire lead-out portions (Japanese Patent Unexamined Publication No. 2003-77730).
When the pair of wires are bifilar-wound while substantially coming into close contact with each other as described in Patent Document 3, normal-mode impedance can be reduced but this is true only in a frequency band sufficiently lower than the cutoff frequency. If the wires come into close contact with each other, there arises a problem that the cutoff frequency is lowered because inter-wire capacitance increases. Particularly in use for a high-speed differential signal interface such as a DVI or an HDMI, a cutoff frequency of about 6 GHz or higher is required. Such a high cutoff frequency cannot be achieved if the pair of wires are just bifilar-wound. Incidentally, in Japanese Patent Unexamined Publication No. 2003-77730, there has been also described a structure in which ends of the wires are connected to the electrodes through electrically conductive junction grooves while separated. Because the wires are led out while coming into close contact with each other in the wire lead-out potions, the problem of reduction in interline voltage proofness and insulation resistance, however, still arises when the electrically insulating films of the wires are deteriorated by heat at the time of connection of the wires. In addition, it is failed to teach about the material of a magnetic core exhibiting good magnetic characteristic in a frequency band of GHz.